The present invention generally relates to a composition comprising a corrosion inhibitor and a method for using the corrosion inhibitor to reduce or stop corrosion of metal from a metallic surface in a corrosive environment. One aspect of the present invention relates more particularly to such a composition and method to reduce or stop corrosion of copper from a circuit board in a solution containing etching agents, such as in a colloidal carbon dispersion, a cleaner, or a conditioner.
In the manufacture of printed circuit boards (PCBs), especially multilayer PCBs, treatment chemicals that can etch copper from the copper surfaces of the circuit boards are employed in several steps. For example, aqueous carbon dispersions or electroless metal deposition baths are used to provide a conductive coating on through holes, via walls and other initially nonconductive surfaces of PCBs. (“Through holes” are holes drilled through double-sided or multilayer circuit boards to complete circuits between the circuit patterns. A “via” as used herein refers either to a through hole or to an open or blind recess, however formed. Vias and through holes can be formed by drilling, by laser or plasma ablation, additively (as by using a photoresist), or in any other way presently known or discovered in the future.) Also, both before and after the PCBs are coated with the aqueous carbon dispersions or electroless metal deposition baths, the PCBs are often treated with cleaners, or conditioners. These aqueous carbon dispersions, cleaners and conditioners commonly contain copper etching agents such as ammonia and monoethanolamine (MEA) that will dissolve copper from the circuit boards into the solution.
Information about such aqueous carbon dispersions, cleaners, conditioners, electroplating baths, and methods for using them, can be found in U.S. Pat. Nos. 5,476,580, 5,389,270, 5,690,805, and 5,725,807 issued to Thorn et al. and U.S. Pat. Nos. 6,375,731 and 6,440,331 issued to Carano, et al. The patents referred to in the preceding sentence are incorporated herein by reference in their entireties. Graphite compositions, cleaners, conditioners, and other materials and directions needed to practice these patents are available under the trademark Shadow® from Electrochemicals Inc., Maple Plain, Minn. Other carbon dispersions containing carbon black, cleaners and conditioners are described, for example, in U.S. Pat. No. 5,139,642, and are available under the trademark Blackhole® from Olin Hunt Specialty Products, Inc. of West Paterson, N.J. This specification may refer to a Shadow® graphite bath or a Blackhole® carbon black bath, while referring to other solutions containing etching agents generally as cleaners, conditioners, electroplating bathes, etc.
A problem with the current manufacture of PCBs is that a Shadow® graphite bath (or a Blackhole® carbon black bath) dissolves copper from the circuit boards contacted by the bath. Because a Shadow® graphite bath is alkaline, it can also react with carbon dioxide (CO2) from the air to form copper carbonate. The copper, copper carbonate or carbonate dissolved in a Shadow® graphite bath causes the bath to form gels. Gel formation leads to a diminished bath life. Cleaners and conditioners can cause similar problems because they also have alkaline etching agents (e.g. MEA) in them and are corrosive solutions. Most cleaners and conditioners used in the PCB industry are dumped on the basis of how much copper is in them.
In other words, in the prior art, copper metal on the circuit boards is attacked by ammonia in the Shadow® graphite solution, or by MEA or other acids or bases in the cleaners or conditioners. The bath becomes ineffective, and then must be disposed of, when it is fouled with excess copper, and new baths are made. This will not only increase manufacturing cost but also raise an environmental issue. Solutions contaminated with copper must be treated before being disposed, and this will certainly incur waste treatment cost.
In the prior art, after the metallic surface of a circuit board is corroded in a cleaner, a conditioner or a Shadow® graphite bath, one would apply a corrosion inhibitor such as benzotriazole (BTA) or tolytriazole (TTA) to the board, and then leave the circuit board in air, tap water, or some other non-corrosive environment, so the metallic surface of the board will not tarnish. One way of doing this is to employ BTA, TTA or mixtures of both in a relatively non-corrosive solution such as water or only mildly acidic water, and apply this solution to the copper surface after it has been treated in a cleaner, a conditioner or a graphite bath and after corrosion has happened. The purpose of this solution is to impart corrosive inhibition to the copper surface after drying. This use of a corrosion inhibitor in prior art is to stop tarnish from forming on the surface of “dry” copper exposed to air while boards are waiting for the next process step. But such use of corrosion inhibitors does not prevent corrosion from happening inside a corrosive environment in the first place. And this use does not prolong the life of a cleaner, a conditioner or a graphite bath.